Purge the question definition cache in Moodle 2.4+.

From Moodle 2.4 onwards, question definitions are cached, so when
we update the question definition in some way that Moodle core does
not know about (for example when deploying variants) we must remember
to notify the question cache, so that it can refresh the definition
of that question.
Update the version number for the STACK 3.0 release.